Pretty much all the life you can see without the help of a microscope–toadstools, poplars, shortstops–is multicellular. Life began as single-celled microbes over 3.5 billion years ago. But at least 25 times over the course of the history of this planet, microbes have come together to form multicellular collectives–otherwise known as bodies.
These transitions are particularly intriguing to evolutionary biologists, because the nature of evolution itself changed along the way. If you’re a microbe, natural selection favors mutations that affect your nature as a single cell. If you stumble across a way to feed on a new nutrient, your descendants may grow faster than the competition and come to dominate your population. Single-celled microbes can evolve to be altruistic–even to commit suicide for the good of their fellow microbe–but they only do so to help their single-celled relatives.
Once microbes evolve a body, they’re playing a new evolutionary game. You have 10 trillion cells in your body, which develop into hundreds of types. Their evolutionary success depends not on their own single-celled reproduction, but the survival of your entire body. A neuron in your brain is not going to go hunting for food. It is going to depend on your body–the brain in which it’s embedded, your muscles, your bones–to find food, on your digestive system to break that food down, and on your circulatory system to deliver it up to the brain. If that whole system works well, it can reproduce. In other words, you have kids.
Scientists suspect that the first step towards a complex multicellular body like ours is for cells to evolve to live in primitive clumps. There may be a lot of advantages to living this way. It may be harder for a predator to eat you, for example. At the University of Minnesota, a team of scientists led by William Ratcliff and Michael Travisano figured out a way to create this kind of natural selection in a lab. As I reported last year in the New York Times, they were able to get yeast–which normally lives as single cells–to turn into simple multicellular clumps in a few weeks.
The scientists reared genetically identical yeast from a single ancestor and then seeded ten flasks of broth with them. The yeast fed on the broth, grew, and budded off daughter cells. The scientists kept the flasks shaking for a day, so that the yeast swirled around. Then they stopped the shaking, and the yeast started to settle down towards the bottom of the flasks. Ratcliff and Travisano drew out a little of that settled yeast and put it in fresh flasks.
And then they repeated the procedure again and again and again. Any mutation that could speed up the yeast’s fall might be favored by natural selection, they reasoned, because fast-falling yeast would be more likely to get scooped up and survive to the next round of the experiment.
In a few weeks, a change did occur. In all ten flasks, the yeast fell faster, forming a cloudy layer at the bottom of the flasks. It was no longer growing as single cells, Ratcliff and Travisano found. In all ten flasks, the yeast was now forming snowflake-shaped clusters, made up of hundreds of cells. The snowflakes formed because daughter cells remained glued to their parents. And the clusters fell faster than their single-celled forerunners.
The cells inside these simple bodies even develop differences. After two months, some of the yeast cells started to commit suicide, while others continued to grow. The dead cells created weak points in the branches, causing them to break off. The broken branches then grew into snowflakes of their own–arguably, a simple form of reproduction.
Obviously, these yeasty snowflakes are a far cry from a human body (or a toadstool, to keep it among the fungi). But it turns out that the evolution of multicellularity didn’t stop with the emergence of these clumps. The initial experiment lasted two months. But then Ratcliff and Travisano extended it nearly five months more. In the journal Evolution, they report that the yeast are still evolving–and that some of the evolution is occurring on the level of their newly-evolved bodies.
Over the course of the new experiment, the yeast continued to adapt to the environment the scientists created for them. By the end of the extra five months, they were falling 45% faster.
They sped up in three different ways. One was by increasing the cells in their bodies. Over the course of the experiment, the number of cells in each cluster nearly tripled, from 42 to 115. More cells made the clusters heavier and made them fall faster.
But after the fourth week of the experiment, the clusters also began to change in a second way: the cells themselves started to swell. By the ninth week of the experiment, the average mass of each cell had doubled. Bigger cells allowed the clusters to fall even faster.
While these changes made the yeasts better fallers, they also came at a cost. Large cells need more energy to reach their greater size. And living in a large cluster makes it hard to for yeast to grow, because the cells buried deep inside the core of a cluster have less food reaching them.
Ratcliff and Travisano found that after two months, the yeast evolution took a third turn, one they suspect was driven by this growing cost. The cells got bigger, but became less dense. Meanwhile, the clusters themselves changed their overall shape.
Originally, the clusters grew in snowflake-like clumps, with branches extended out on all sides. Ratcliff and Travisano found that after two months, the clumps grew more spherical. Analyzing these ball-shaped clumps, the scientists found that they fell faster than branched clumps with the same number of cells. That’s likely because the yeast could slip through the water more efficiently, without so many branches creating drag.
These changing shapes were not the result of identical adaptations occurring in each cell. They were changes to the overall anatomy of the clusters. Along with the emergence of the branch-snapping suicidal cells, this shape change demonstrates that evolution has shifted in Ratcliff’s experiments. It’s moved from the single cell to the proto-body. And as the proto-body grows and faces new constraints, it is evolving new solutions for many cells living together.
There may well be more strategies for getting bigger that the yeast have yet to evolve. I don’t expect man-sized yeast creatures walking out of Ratcliff and Travisano’s lab any time soon. But I am curious what another year or two of evolution will reveal.
(PS: When I wrote about this research last year, Ratcliff offered some responses to questions raised on the blog and on Twitter. For those curious for more details about this research, check them out.)